1. Field of the Invention
The present invention relates generally to a semiconductor device. More specifically, the invention relates to a method for depositing a tungsten silicide film for use in a gate electrode/wiring (a gate electrode and/or a wiring), and a method for preparing a gate electrode/wiring.
2. Description of Related Background Art
In semiconductor devices, such as LSIs, a so-called polycide structure comprising a polysilicon layer 10 and a tungsten silicide layer 20 stacked thereon as shown in FIG. 3 is widely used as a gate electrode and/or a wiring in order to lower the resistance of the gate electrode and wiring. The tungsten silicide layer serving as a top layer of the polycide structure is generally deposited by the CVD method using WF6/SiCl2H2/Ar as a reactive gas. Therefore, in the conventional thin-film deposition method, a desired film quality of tungsten silicide layer is obtained by controlling the deposition temperature, the pressure of the reactive gas, and the flow rate and flow ratio of the gas. In FIG. 3, reference number 30 denotes a silicon substrate, 31 denotes a silicon oxide film, 40 denotes a silicon oxide film, 50 denotes a silicon nitride film, and 60 denotes a side wall of a silicon oxide film.
However, in the present circumstances, if an electrode and/or wiring having a polycide structure, which is scaled down as small as possible, is prepared by a conventional method for depositing a tungsten silicide film, there are some cases where a void V is formed in the electrode and/or wiring in the producing stage as shown in FIG. 3. Although the void has little influence on the productivity of semiconductor devices in the present stage, it is predicted that the influence of such a void becomes obvious to lower yields if the scale down and lowering of resistance of the electrode and/or wiring further proceed in future.
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide a method for depositing a tungsten silicide film and a method for preparing a gate electrode/wiring, which can enhance yields in future without forming any voids in the electrode and/or wiring having a polycide structure. It is another object of the present invention to provide a gate electrode/wiring structure which is prepared by the gate electrode/wiring preparing method according to the present invention.
The inventors found that depressions Vxe2x80x2 corresponding to voids existed in the central portion of the surface of a polysilicon layer 10 when a tungsten silicide layer 20 was peeled off from the polysilicon layer 10 after a side wall oxidizing step of an electrode/wiring preparing process was completed in order to identify a void occurring step (see FIG. 4). On the basis of this, the inventors predicted that voids occur at the side wall oxidizing step by trying the identification of void existing places and existing step, and carried out the side wall oxidizing process while varying process temperature and process time. As a result, the inventors found that voids occurred at a high process temperature although no void occurred when the process time was short and/or when the process temperature was low. From this fact, the inventors presumed that since more lattice defects and interstitial atoms concentrate on the interface between the polysilicon layer 10 and the tungsten silicide layer 20 than other portions, silicon atoms in the surface of the polysilicon layer 10 use lattice defects as media to diffuse to be consumed for the side wall oxidation to produce voids, as shown in FIG. 3 by an arrow, when the side wall oxidizing step is carried out at a high temperature. The side wall oxidizing step means a step of forming an oxide film on the side wall of the tungsten silicide layer 20 and on the side wall of the polysilicon layer 10 in order to prevent impurities from being injected into the tungsten silicide layer 20 and the polysilicon layer 10 from oblique directions when the impurities are ion-implanted.
Therefore, the inventors studied various tungsten silicide film depositing methods and gate electrode/wiring preparing methods which do not fear that silicon atoms are consumed even at the side wall oxidizing step. As a result, the inventors knew that the above described objects of the present invention can be accomplished by depositing a tungsten silicide film or preparing a gate electrode/wiring on specific conditions.
The present invention has been made on the basis of the above described knowledge. In order to accomplish the aforementioned and other objects, according to a first aspect of the present invention, there is provided a method for depositing a tungsten silicide film, wherein when a tungsten silicide layer is formed on a polysilicon layer, a phosphorus atom containing gas is added to a reactive gas at least in the initial stage that the tungsten silicide layer is formed, and the amount of the added phosphorus atom containing gas is set to be in the range of from 0.2 vol. % to 0.45 vol. %.
According to a second aspect of the present invention, there is provided a method for depositing a tungsten silicide film, wherein when a tungsten silicide layer is formed on a polysilicon layer, a phosphorus atom containing gas is added to a reactive gas at least in the initial stage that the tungsten silicide layer is formed, and a tungsten silicide layer forming temperature is set to be a temperature at which silicon atoms of the polysilicon layer are activated.
In the second aspect of the present invention, the tungsten silicide layer forming temperature is preferably set to be at least 700xc2x0 C.
In the first and second aspects of the present invention, the method preferably includes a first stage in which the phosphorus atom containing gas is added to the reactive gas, and a second stage in which the phosphorus atom containing gas is not added to the reactive gas.
According to a third aspect of the present invention, there is provided a method for preparing a gate electrode/wiring, which comprises the steps of depositing a tungsten silicide layer on a polysilicon layer, and depositing a silicon layer on the tungsten silicide layer.
According to a fourth aspect of the present invention, there is provided a method for preparing a gate electrode/wiring, which comprises a step of depositing a tungsten silicide layer on a polysilicon layer, a step of oxidizing a side wall of a gate electrode/wiring layer containing the polysilicon layer and the tungsten silicide layer, and a short-time annealing step carried out between the depositing and oxidizing steps.
According to a fifth aspect of the present invention, there is provided a gate electrode/wiring structure which comprises a polysilicon layer, a tungsten silicide layer formed on the polysilicon layer, and a silicon layer formed on the tungsten silicide layer.
The tungsten silicide film depositing method (which will be hereinafter referred to as a xe2x80x9cdeposition methodxe2x80x9d) according to the present invention is characterized in that a tungsten silicide layer is formed on a polysilicon layer, which has been previously formed by a conventional well-known method, by a method which will be described later. According to the present invention, the phosphorus atom containing gas is a gaseous molecule bonded to phosphorus atoms, and is preferably phosphine (PH3). The reactive gas is a mixed gas composition comprising various gases required when tungsten silicide is produced, and is preferably a mixed gas of tungsten hexafluoride (WF6), dichlorosilane (SiCl2H2) and argon (Ar). The composition ratio of the gases (WF6/SiCl2H2/Ar) can be suitably set.
In the deposition method according to the present invention, a gas composition, which comprises a reactive gas and a phosphorus atom containing gas added to the reactive gas at least in the initial stage that a tungsten silicide layer is formed, is used when a tungsten silicide layer is formed on a polysilicon layer. By adding the phosphorus atom containing gas to the reactive gas, the growth nucleus of tungsten silicide can be formed in the surface of the polysilicon layer to grow a tungsten silicide crystal on the basis of the growth nucleus to suitably control the crystal grain and crystal orientation of the tungsten silicide film during the growth, so that it is possible to obtain a film quality having a small specific resistance and an excellent migration resistance. According to the present invention, phosphorus atoms are not only added, but the amount of the added phosphorus atom containing gas is also set to be in the range of from about 0.2 vol. % to about 0.45 vol. % so that it is possible to prevent silicon atoms of the polysilicon layer from diffusing. By setting the amount of the added phosphorus atom containing gas to be this range, lattice defects in the top face or vicinity of the polysilicon layer can be filled with phosphorus atoms to prevent or inhibit silicon atoms from diffusing from the polysilicon layer. If the amount of the added phosphorus atom containing gas is less than 0.2 vol. % of the reactive gas, it is not possible to obtain the effect that the phosphorus atom containing gas is added, and if the amount of the added phosphorus atom containing gas exceeds 0.45 vol. %, there is the possibility that the film quality deteriorates.
Therefore, since the phosphorus atom containing gas may be added at least in the initial stage that the tungsten silicide layer is formed, the phosphorus atom containing gas may or may not remain being added after the initial stage until the deposition step is completed. In either case, the deposition step is preferably divided into a first stage and a second stage. In the first stage, a predetermined amount of phosphorus atom containing gas is added, and in the second stage, a smaller amount of phosphorus atom containing gas than that in the first stage is added, or no phosphorus atom containing gas is added.
In the first stage, a first tungsten silicide film of a tungsten silicide layer containing relatively rich silicon is preferably formed. At this step, the reactive gas, to which the phosphorus atom containing gas has been added, is used as described above. By forming the growth nucleus of tungsten silicide using this gas, the crystal grain size and crystal orientation can be controlled. The tungsten silicide layer containing relatively rich silicon means a layer containing a high proportion of silicon atoms so that the ratio of atomic numbers (x/y) of tungsten silicide (WxSiy) is less than 2/5.
The second stage is carried out continuously after the first stage. At this step, a second tungsten silicide film of a tungsten silicide layer containing relatively rich tungsten is preferably formed. At this step, the tungsten silicide film can be deposited on the basis of the growth nucleus which has been formed in the first stage. Therefore, in the second stage, a tungsten silicide layer containing relatively rich tungsten, which has uniform crystal grain size and crystal orientation, can be formed only using the reactive gas without the need of the phosphorus atom containing gas. The tungsten silicide layer containing relatively rich tungsten means a layer containing a high proportion of tungsten atoms so that the ratio of atomic numbers (x/y) of tungsten silicide (WxSiy) exceeds 2/5. Since the second stage is carried out continuously from the first stage, there are some cases where phosphorus atoms contained in the reactive gas used in the first stage remain even if the reactive gas, to which no phosphorous atom containing gas has been added, is used in the second stage. For that reason, there are some cases where the remaining phosphorus atoms in the first stage is mixed in the reactive gas containing no phosphorus atom, which is used in the second stage, to form a tungsten silicide film containing a very small amount of phosphorus atom although phosphorous atoms are not added in the second stage. There are also some cases where even if the reactive gas containing no phosphorus atom is used in the second stage that the tungsten silicide film is deposited, phosphorus atoms diffuse from the first tungsten silicide film (lower layer) to the second tungsten silicide film (upper layer), so that the tungsten silicide film containing phosphorus atoms is formed.
When the reactive gas, to which the phosphorus atom containing gas has been added, is used even in the second stage, the proportion of the added phosphorus atom containing gas in the second stage is set to be lower than that in the first stage. Thus, the upper layer is made of a tungsten silicide film containing a lower concentration of phosphorus atoms than that of the lower layer. By thus setting the concentration of phosphorus in the second tungsten silicide film serving as the upper layer to be lower than the concentration of phosphorus in the first tungsten silicide film serving as the lower layer, phosphorus atoms diffuse from the lower layer to the upper layer during the heat treatment after the deposition, so that it is possible to lower the concentration of phosphorus in the lower layer. The phosphorus atoms in the lower layer diffuse into the upper layer after the heat treatment, so that it is possible to finally obtain a tungsten silicide film having a substantially uniform concentration of phosphorus in whole.
In the deposition method according to the present invention, the tungsten silicide film forming temperature (which will be hereinafter referred to as a xe2x80x9cdeposition temperaturexe2x80x9d) is set to be a temperature at which silicon atoms of the polysilicon layer are activated. This temperature is preferably set to be at least 700xc2x0 C. Since silicon atoms of the polysilicon layer and tungsten silicide layer are thus activated, lattice defects are filled with silicon atoms immediately even if the lattice defects are produced, so that no voids are produced.
For that reason, even if thermal energy is applied at the subsequent side wall oxidizing step, the diffusion of silicon atoms derived from lattice defects does not occur between the polysilicon layer and the tungsten silicide layer to decrease the absolute amount of silicon atoms consumed by the side wall oxidation, so that it is possible to prevent voids from being formed. Therefore, when the deposition temperature is set to be a high temperature of 700xc2x0 C., the amount of phosphorus atoms is sufficient to be an amount capable of controlling the crystal grain size and crystal orientation of the tungsten silicide layer, e.g., in the range of from 0.02 vol. % to 0.2 vol. %.
The gate electrode/wiring preparing method according to the present invention includes a step of depositing a tungsten silicide layer on a polysilicon layer, and a step of depositing a silicon layer on the tungsten silicide layer. Although the former tungsten silicide film forming step can use a conventional well-known deposition method, the above described deposition method according to the present invention is preferably used in order to cope with the scale down and lowering of resistance of the electrode and/or wiring. The latter silicon film deposition step can use a conventional well-known deposition method. This silicon layer serves as a silicon atom supply source for a silicon oxide film which is formed at the side wall oxidizing step. Since the silicon atoms of the silicon layer are substituted for the silicon atoms of the polysilicon layer, the silicon atoms of the silicon layer are preferentially supplied for the silicon oxide film at the side wall oxidizing step even if lattice defects exist between the polysilicon layer and the tungsten silicide layer. By the silicon atoms of the silicon layer, the polysilicon atoms concerning the lattice defects of the polysilicon layer are inhibited or prevented from diffusing the surrounding, so that voids are inhibited or prevented from being formed between the polysilicon layer and the tungsten silicide layer.
Another gate electrode/wiring preparing method according to the present invention includes a step of forming a tungsten silicide layer on a polysilicon layer, a step of oxidizing a side wall of a gate electrode/wiring layer including the polysilicon layer and the tungsten silicide layer, and a short-time annealing step carried out between these steps. Although the former step of forming the tungsten silicide layer on the polysilicon layer can use a conventional well-known deposition method, the above described deposition method according to the present invention is preferably used. If the conventional well-known tungsten silicide film depositing method is used, lattice defects concentrate between the polysilicon layer and the tungsten silicide layer. Therefore, according to the present invention, the short-time annealing step is introduced between the deposition step and the side wall oxidizing step. By introducing the short-time annealing step after forming the tungsten silicide layer, the heat treatment activates the respective atoms of the polysilicon layer and tungsten silicide layer to diffuse the atoms into each other even if lattice defects concentrate on the interface between the polysilicon layer and the tungsten silicide layer, and particularly, the lattice defects existing in the interface are filled with silicon atoms to correct the lattice defects. By the short-time annealing, it is possible to obtain the same effects as those when a temperature (a high temperature of at least 700xc2x0 C.), at which polysilicon atoms are activated, is set in the stage that the tungsten silicide layer is formed.
Therefore, even if the side wall oxidation is carried out after the short-time annealing, part of silicon atoms near the interface between the polysilicon layer and the tungsten silicide layer do not intensively diffuse into the surrounding, and the silicon atoms of the polysilicon layer near the interface diffuse uniformly on the whole surface to be supplied to the side wall oxide film, so that the thickness of the polysilicon layer can be decreased as a whole to prevent voids from being produced. The short-time annealing is carried out in, e.g., an inert gas atmosphere, such as nitrogen gas, by applying thermal energy from a heat source, such as a halogen lamp.